A CELLULAR FORMS (Viruses & Bacteriophages) A cellular forms, most range in size from 5 to 300 nanometers (nm) * in diameter, although some Paramyxoviruses can be up to 14,000nm long. They are usually so small that they readily pass through a very fine porcelain filter or asbestos discs. Two types recognized to belong to this forms : Viruses. Bacteriophages. __________________________________________ ________ *(n= Nano =10 -9 m ; µ= micro =10 -6 m; m= milli =10 -3 m)

What is a virus? Viruses may be defined as acellular organisms whose genomes consist of nucleic acid, and which obligately replicate inside host cells using host metabolic machinery and ribosomes to form a pool of components which assemble into particles called VIRIONS, which serve to protect the genome and to transfer it to other cells. defined acellular organisms genomes

Viruses are obligate intracellular parasitic microorganisms which are smaller than bacteria and so could pass through bacterial filters. They form a group which live on other living systems such as humans, animals, insects, plants, bacteria and fishes. The largest ones coming somewhat closer to the smallest bacterium. These viruses are highly selective and specific to their hosts. They are known to be responsible for several diseases of man, animal, plant, etc.

Viruses Viruses are infectious agents with both living and nonliving characteristics. 1. Living characteristics of viruses a. They reproduce at a fantastic rate, but only in living host cells. b. They can mutate. 2. Nonliving characteristics of viruses a. They are acellular, that is, they contain no cytoplasm or cellular organelles. b. They carry out no metabolism on their own and must replicate using the host cell's metabolic machinery. In other words, viruses don't grow and divide. Instead, new viral components are synthesized and assembled within the infected host cell. c. The vast majority of viruses possess either DNA or RNA but not both.

Nature of viruses Viruses cannot be grown on artificial media. They can only grow in living organisms or tissue cells which are kept alive in suitable medium.

Morphology of Virus According to the shape, viruses can be classified into the following groups : Spherical. With a size ranging from 18 to 150 m μ This includes, as example, the viruses of influenza. Rod-shaped. They are 300 mμ in length and 15 mμ in width. It is represented by the tobacco mosaic virus. Cuboid. With a size ranging from 210 to 305 mμ. This form is found in cowpox and canary pox viruses (Fig. 26 C ). Spermatozoid-shaped. The size varies from 10 to 225 mμ. This form is characteristic of phages.

Composition of viruses Viruses have the same general chemical characteristics, being composed of molecules of nucleic acid ( either RNA or DNA ) and protein. They have the capacity of existing either alone or aggregated in the form of crystals which contain millions or even billions of molecules. The virus unit or particle is called a virion. The virion structure is similar in different virions from the general point of view. This virion is composed of folded strands of nucleic acid inside a coat made of regularly arranged protein subunits (capsoids). The simple types of viruses are only nucleoproteins, while the most complex types (as cowpox) contain in addition other compounds such as lipids, carbohydrates and sometimes traces of metals and vitamin-like substances. Viruses contain either DNA or RNA, but both never occur together in one virus. The plant viruses contain only RNA while the animal viruses may contain either RNA or DNA. Bacterial viruses or phages usually contain DNA. Compassion of virus

Mechanism of virus infection ( Reproduction ) 1. Adsorption of the virus : The infective virus has to be bound at first to certain receptors on the outer surface of the host cell. 2. Penetration: There are two views concerning this phase :  The whole virus consisting of nucleic acid and protein penetrates into the cell, although the protein part has no role on the further steps of infection.  Only the nucleic acid part penetrates into the cell, i.e. the virus has to get rid of its protein coat before penetration 3. Blocking of cell information :The introduction of viral nucleic acid in the host cell inhibits and blocks the original genetic information in the DNA of the chromosomes, and the nucleus has no longer any control on the cell activity. 4. Synthesis of virus components. the viral nucleic acid forces the cell to replicate the virus constituents ( nucleic acid and protein ). This new synthesis is completed in few hours. 5. Excretion of the virus : In some virus diseases the excretion or release of the virus is accompanied by the disintegration of the cell, while in others both the nucleus and cytoplasm remain contact.

life cycle of virus